Currently, it is apparent that OLED assemblies are already now of economic significance, since mass production is expected shortly. Such OLEDs consist predominantly of organic layers which can also be manufactured flexibly and inexpensively. OLED components can be configured with large areas as illumination bodies, but also in small form as pixels for displays.
Compared to conventional technologies, for instance liquid-crystal displays (LCDs), plasma displays or cathode ray tubes (CRTs), OLEDs have numerous advantages, such as a low operating voltage of a few volts, a thin structure of only a few hundred nm, high-efficiency self-illuminating pixels, high contrast and good resolution, and the possibility of representing all colors. In addition, in an OLED, light is produced directly on application of electrical voltage, rather than merely being modulated.
A review of the function of OLEDs can be found, for example, in H. Yersin, Top. Curr. Chem. 2004, 241, 1 and H. Yersin, “Highly Efficient OLEDs with Phosphorescent Materials”; Wiley-VCH, Weinheim, Germany, 2008.
Since the first reports regarding OLEDS (see, for example, Tang et al., Appl. Phys. Lett. 1987, 51, 913), these devices have been developed further particularly with regard to the emitter materials used, and particular interest has been attracted in the last few years by so-called triplet emitters or other phosphorescent emitters.
OLEDs are generally implemented in layer structures. For better understanding, FIG. 1 shows a basic structure of an OLED. Owing to the application of external voltage to a transparent indium tin oxide (ITO) anode and a thin metal cathode, the anode injects positive holes, and the cathode negative electrons. These differently charged charge carriers pass through intermediate layers, which may also consist of hole or electron blocking layers not shown here, into the emission layer. The oppositely charged charge carriers meet therein at or close to doped emitter molecules, and recombine. The emitter molecules are generally incorporated into matrices consisting of small molecules or polymer matrices (in, for example, 2 to 10% by weight), the matrix materials being selected so as also to enable hole and electron transport. The recombination gives rise to excitons (=excited states) which transfer their excess energy to the respective electro-luminescent compound. This compound can then be converted to a particular electronic excited state which is then converted very substantially and with substantial avoidance of radiationless deactivation processes to the corresponding ground state by emission of light.
With a few exceptions, the electronic excited state, which can also be formed by energy transfer from a suitable precursor exciton, is either a singlet or triplet state. Since the two states are generally occupied in a ratio of 1:3 on the basis of spin statistics, the result is that the emission from the singlet state, which is referred to as fluorescence, according to the present state of the art, leads to maximum emission of only 25% of the excitons produced. In contrast, triplet emission, which is referred to as phosphorescence, exploits and converts all excitons and emits them as light (triplet harvesting), such that the internal quantum yield in this case can reach the value of 100%, provided that the additionally excited singlet state which is above the triplet state in terms of energy relaxes fully to the triplet state (intersystem crossing, ISC), and radiationless competing processes remain insignificant. Thus, triplet emitters, according to the current state of the art, are more efficient electro-luminophores and have better suitability than purely organic singlet emitters for ensuring a high light yield in an organic light-emitting diode.
In the triplet emitters suitable for triplet harvesting transition metal complexes are generally used in which the metal is selected from the third period of the transition metals. This predominantly involves very expensive noble metals such as iridium, platinum or else gold. (See also H. Yersin, Top. Curr. Chem. 2004, 241, 1 and M. A. Baldo, D. F. O'Brien, M. E. Thompson, S. R. Forrest, Phys. Rev. B 1999, 60, 14422).
The phosphorescent organometallic triplet emitters known to date in OLEDs, however, have a disadvantage, which is that these complexes are frequently more chemically reactive in electronically excited states than in the base states. Responsible for this are generally metal-ligand bond breakages. Therefore, the long-term stability of these emitter materials is inadequate in many cases. (T. Sajoto, P. I. Djurovich, A. B. Tamayo, J. Oxgaard, W. A. Goddard III, M. E. Thompson; J. Am. Chem. Soc. 2009, 131, 9813). As a result, efforts are being made to develop emitter molecules without metal centers and with high emission quantum yield, wherein the emitter molecules shall furthermore also convert all singlet and triplet excitons into light. OLEDs using such emitters should exhibit a high efficiency, and additionally enable a longer lifetime of the optoelectronic device.
In summary, the prior art can be described such that the triplet emitters which are efficient per se and are known to date have the disadvantages that                expensive noble metal molecules have to be used and that        these emitters formed on the basis of organometallic complexes have only inadequate long-term stability in many cases.        